Many security systems use radiation from light emitting diodes or laser diodes or other laser sources for illuminating objects of interest. In particular, laser designator systems are used to designate targets for many military and law enforcement applications. For a typical laser designator system, a near infrared (NIR) 1064 nm laser diode is often used. The laser is normally modulated at, for example, pulse durations as short as 10 ns, pulse repetition rates as high as 20 kHz, and with as little as 25 μJ per pulse. Various conventional focal plane array (FPA) technologies can be used for detecting the 1064 nm laser from a laser designator system. Typically, the laser provides a laser spot on the target and an FPA detects the laser spot with an imager such as an indium gallium arsenide (InGaAs) imager, a mercury cadmium telluride (HgCdTe) imager, a thick-epi CMOS (Complementary metal-oxide-semiconductor) imager, or a charge-couple device (CCD).
A problem with using a conventional imager to provide a see-spot capability for a laser designator system is capturing the reflected laser pulse energy. In order for the imager to capture the reflected laser pulse, the imager must be gated in time to coincide with the time of arrival of the reflected laser pulse from the designated target. By gating the imager to “detect” the laser spot and not allowing the charge wells to charge except when the laser return is expected, the imager sacrifices all surrounding (background) video imagery. In other words, the only thing that is often seen in the video frame is the laser spot itself, and imagery of any surrounding scenery is not captured. A resultant scene is often too dark to discern any details except for the spot because the charge wells within the FPA did not receive sufficient photons from the surrounding scenery to produce a useful image due to the limited gate time allotted to the laser pulse. In order to overcome this phenomenon, a separate sensor is normally used in the system capable of capturing the normal scene. The image output from those two separate sensors is digitally merged or fused into one composite image. Since the field-of-view (FOV) is not exactly same for the two sensors, there will be registration errors. This could pose serious problems for applications where high accuracy of aiming is desired.